xen: install xen/gntdev.h and xen/gntalloc.h

xen/gntdev.h and xen/gntalloc.h both provide userspace ABIs so they
should be installed.

CC: stable@vger.kernel.org
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>